All national security systems have their methods of keeping track of subversive activity. No doubt a certain number of members of subversive organizations are also police agents, and no doubt a number of police agents are also members of subversive organizations. This is as likely to have been as true of Tsarist Russia as of anywhere else. Certainly the penetration of police agents into the revolutionary movement and of revolutionaries into the secret police was carried to such lengths that in a number of well known cases it was impossible to tell whose interests were being served by certain individuals. In this article I describe four such cases—E. F. Asev, organizer of the assassination of the Russian Minister of the Interior in 1904 and of the Grand Duke Sergius in 1905; Father G. A. Gapon, the priest who led the procession of workers on “Bloody Sunday” in 1905; Bogrov, the assassin of the Russian Prime Minister in 1911; and R. V. Malinovsky, who was leader of the Bolsheviks in the Duma from 1912 until 1914.